Circular logic, as Jesus had no blood brothers yet Scripture seems to refer to them.
I grant that, if ‘brother’ is used differently when referring to the brothers of Jesus than it is anywhere else, then you can be right. However, if a word is used one way every time it is used EXCEPT in the time in dispute, then it is up to the person claiming the exception to show why an exception should be made.
For a Catholic, the answer is (I believe), my doctrine requires it.
And that is a valid answer, since my point is not that you must believe Jesus had brothers to be a Christian. I’m certain a great many true believers have gone to the Lord convinced of Mary’s perpetual virginity. I’m not convinced they retain that belief afterward, but I’ll find that answer out for myself someday...and Mr Rogers has been known to be wrong.
It boils down to the same thing many of the disputes between Protestants and Catholics do - does scripture drive your doctrine, or does doctrine ‘interpret’ the scripture?
I assume the former, and Catholics assume the latter - boast of it, in fact. Therefore, we come to different conclusions.